A significant use of hot rolled steel bar is in coil and torsion bar suspension springs employed in passenger cars and light trucks. Manufacturers of these vehicles are placing greater requirements on suspension systems than has previously been the case. Vehicles weight reduction, size constraints, handling, performance and styling needs all impacting on the springs design. The two most significant requirements for coil and torsion bar springs are the need for smaller size or "package" and reduced weight. Package refers to the ability of the design to fit under increasingly lower engine hood lines and into shorter chassis frames and to allow increases in the available space passenger and cargo areas. In this regard, new suspension springs must be increasingly smaller than current designs. The desired weight reduction is an accompanying benefit of a smaller spring.
In terms of size and weight, a smaller spring translates into a steel bar of generally decreased diameter and length. These reductions will result in higher working stresses in the spring for the same load and spring rate. The inventors herein have developed a steel composition from which springs may be formed and which meets the size and weight needs while maintaining or enhancing spring performance, i.e. fatigue behavior and sag resistance.
The applicants are aware of certain scientific literature and prior patents relating to spring steel compositions and of certain commercially-available steel grades. In particular, U.S. Pat. No. 4,409,026 describes a spring steel composition for automobile use comprising 0.5 to 0.7 wt% C, 1.0 to ? .8 wt% Si, 0.1 to 1.0 wt% of Mn, below 0.7 wt% Cr, 0.03 to 0.5 wt% V and the balance iron and normally present impurities, and optionally at least one of Al, Zr, Nb and Ti, each contained in an amount of 0.02 to 0.I wt%. Accordingly, a critical combination of defined amounts of C, Si, Mn, Cr and V is required for this composition.
U.S. Pat. No. 4,574,016 describes a steel exhibiting good sag resistance and useful in a vehicle suspension spring comprising 0.5 to 0.80 wt% C, 1.50 to 2.50 wt% Si, 0.50 to 1.50 wt% Mn, plus 0.05 to 0.50 wt% V, 0.05 to 0.50 wt% Nb or 0.05 to 0.50 wt% Mo, with the remainder being iron together with impurities. The steel may further contain a member or members selected from 0.0001 to 0.01 wt% B, 0.2 to 1.00 wt% Cr and not greater than 0.0008 wt% N. Again, a critical combination of defined amounts of C, Si, Mn and V (or Nb or Mo) is required for this composition.